submission: Move some headers and add a note

docs/02-for-app-authors/05-submission.md

@@ -1,6 +1,8 @@
 # Submission
 
 App submissions are extremely welcome and the process is straightforward.
+Some basic familiarity with Git and Flatpak is required to submit and
+maintain an app on Flathub.
 
 ## Before submission
 
@@ -10,6 +12,8 @@ Please follow the [requirements](/docs/for-app-authors/requirements) to
 ensure that the submission has all the required files and is technically
 and legally compatible for Flathub.
 
+### Build and install
+
 It's best practice to make sure your submission correctly builds and runs
 locally with Flatpak. This will help you to fix any potential issues and
 expedite the review process.
@@ -21,8 +25,6 @@ to build the application.
 flatpak install -y flathub org.flatpak.Builder
 ```
 
-### Build and install
-
 Add the Flathub repo user-wide:
 
    ```bash